Open AccessThe coherence of kilohertz quasi‐periodic oscillations in the X‐rays from accreting neutron stars
Author(s) -
Barret Didier,
Olive JeanFrancois,
Miller M. Coleman
Publication year - 2006
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2006.10571.x
Subject(s) - physics , neutron star , astrophysics , stars , luminosity , amplitude , quality (philosophy) , astronomy , optics , quantum mechanics , galaxy
We study in a systematic way the quality factor of the lower and upper kilohertz quasi‐periodic oscillations (kHz QPOs) in a sample of low‐luminosity neutron star X‐ray binaries, showing both QPOs varying over a wide frequency range. The sample includes 4U 1636−536, 4U 1608−522, 4U 1735−44, 4U 1728−34, 4U 1820−303 and 4U 0614+09. We find that all sources except 4U 0614+09 show evidence of a drop in the quality factor of their lower kHz QPOs at high frequency. For 4U 0614+09 only the rising part of the quality factor versus frequency curve has been sampled so far. At the same time, in all sources but 4U 1728−34, the quality factor of the upper kHz QPO increases all the way to the highest detectable frequencies. We show that the high‐frequency behaviours of both the lower and the upper kHz QPO quality factors are consistent with what is expected if the drop is produced by the approach of an active oscillating region to the innermost stable circular orbit: the existence of which is a key feature of general relativity in the strong field regime. Within this interpretation, our results imply gravitational masses around 2 M ⊙ for the neutron stars in those systems.